1. Field of the Invention
The present invention relates to a process for the preparation of dialkyl carbonates by reaction of carbon monoxide (CO) with alkyl nitrites in the presence of a platinum metal halide catalyst on aluminium oxides, aluminium oxide hydrates or aluminium hydroxides as support which may contain additives of compounds of further elements. The aforementioned supports are characterised by a BET surface area of more than 1 m.sup.2 /g.
Dialkyl carbonates are of general chemical importance. Thus, for example, diethyl carbonate is an excellent solvent in the medium boiling range. Furthermore, dialkyl carbonates are excellent carbonylating and acylating agents. Finally, they are of great importance in the preparation of other carbonates, of urethanes and of ureas.
2. Description of the Related Art
The preparation of dialkyl carbonates by reactions of phosgene or alkyl chloroformates with alcohols is known. However, there is an increasing interest in replacing the use of the toxic phosgene or the intermediates derived therefrom, such as the chloroformic ester, by other processes. In addition to attempts to obtain dialkyl carbonates by reaction of CO with lower alcohols, in particular those processes are important in which CO is reacted in the gas phase with alkyl nitrite over a platinum metal catalyst. In reactions of this type, the formation of dialkyl oxalate is always observed apart from the desired dialkyl carbonate. Thus, EP 425,197 discloses a process which, according to its preferred embodiment, results in dialkyl carbonates of methanol or ethanol from CO and methyl nitrite or ethyl nitrite in the gas phase over a PdCl.sub.2 catalyst on activated carbon. The selectivities to give the desired lower dialkyl carbonates reach values of up to 94% according to this EP 425,197, Table 1; however, lower dialkyl oxalates and CO.sub.2 are always observed as by-products. Moreover, upon attempting to carry this process in practice, the high selectivities mentioned could only be reproduced insufficiently. The catalysts of this EP 425,197 contain additives of chlorides of base metal; a substantial addition of hydrogen chloride in an amount of 1 to 50%, relative to the platinum metal in the catalyst, is added to the system or a portion of the catalyst must be removed from the reactor and subjected to a treatment with hydrogen chloride.
The Journal for Catalytic Research (China), Vol. 10 (1), p. 75-78 (1989) also uses a carbon support as support for a Pd catalyst so as to obtain dimethyl carbonate from CO and methyl nitrite, in which however dimethyl oxalate is always additionally formed.
A Pd/carbon catalyst is also mentioned in Chin. Sci. Bull. 34 (1989), 875-76 for the preparation of dimethyl carbonate from CO and methyl nitrite.
This preference for a carbon support is not unexpected, since Platinum Metals Review 34 (1990), 178-180 reports with reference to the earlier literature that in the reaction of a lower alkyl nitrite with CO over a Pd catalyst different main products are obtained depending on the support; according to this reference a carbon support predominantly produces dialkyl carbonates, while an Al.sub.2 O.sub.3 support mainly produces dialkyl oxalates.